There is a consensus that wireless data and multimedia traffic will overtake voice traffic in a relatively short span of time (perhaps as early as 2002). Cellular technologies are developing at a very rapid pace. As outlined elsewhere in this summary report of the panelists, new techniques have been developed for the third generation (3G) ranging from radio frequency components, antenna technology, signal processing, source and channel coding, interference reduction, and the various methods for improving spectral efficiency. Virtually all of these novelties and inventions are directed at improving the air interface, i.e., what happens between a mobile user and a base station at radio frequency transmission, reception and subsequent signal processing. However, for the most part, the large-scale (core) network that ultimately connects a wireless user to other remote users (wireless or otherwise) is still based on the traditional circuit switched network designed to carry telephonic voice traffic. One of the major dilemmas facing network designers is how to make a convergence among four apparently different service objective as shown in Fig. 3.1.
Figure 3.1 shows on the lower left the historic basis of communication networks, i.e., (voice) telephony. Traditionally, and for the most part, using a wired network. Furthermore, and perhaps more importantly, the talking path and end to end connection is circuit switched with dedicated resources being allocated (trunk lines, switching, monitoring subroutines, etc.) exclusively for each call. While call set up has progressed to a (logically) separate packet switched data network (SS7), the concepts remain rooted in the dominance of fixed location telephony. The wired network started to be used for the transmission and reception of non-voice data traffic over 30 years ago using modems, and more recently, with ISDN, ADSL, cable systems and even fixed point satellite access. These, and the older transmission means, are now providing multimedia and Internet traffic to homes and offices. This service development is the third bubble on the lower right of Fig. 3.1. Finally, the upper left bubble is wireless technology that allows tetherless and roaming access to users.
Fig. 3.1. Four way convergence.
For the most part, as stated earlier, wireless services have been, until very recently, mostly telephony and short message services (paging, etc.). The core mechanism for switching and routing of wireless services remains dependent on the telephony circuit switching even for the third generation (3G) of wireless network. It is a prime objective of many organizations in Europe, North America, Asia, and elsewhere to converge all these and possibly future service on a common platform. In this report, we review some of the ideas and plans to accomplish that. A fairly conservative approach currently being deployed is shown in Fig. 3.2.
Fig. 3.2. Vision of a seamless wireless network.
The objectives of such a plan is to "marry" the circuit switched cellular wireless systems shown on top, with the IP (Packet switched) Internet network via gateways. Such a solution could include first and second generation wireless or other legacy air interfaces, but it is wholly insufficient for the kind of features required for mixed media and broadband services. These features include, but are not limited to the following:
Roaming capability is provided for in 2nd generation systems by transferring the database relating to a roaming user from a home location register (HLR) to a visitor location register (VLR) over a signaling network. An illustration of this for GSM is shown in Fig. 3.3 where the standard B, C, D, and G interfaces are used to link the VLR to the HLR and the Mobile (circuit) Switching Centers (MSC) to each other.
Fig. 3.3. Second generation (GSM/CDMA) network architecture.
The Equipment Identification Register (EIR) is used to verify and identify the individual mobile user equipment. CDMA also uses a similar kind of architecture with the main differences being the air interface modulation and the interface between the Base Station (BS) and the Base Station Controller (BSC), Abis. As shown in Fig. 3.3 there are also provisions for the MSC to connect directly to the Public Switched Telephone Network (PSTN) via digital lines or to use ISDN.